This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 00 13977 filed in France on Oct. 31, 2000; the entire content of which is hereby incorporated by reference.
The invention relates to a noise reduction sandwich panel of the type including a waffle core mounted between a porous resistive layer and a reflector.
Such a sandwich panel may advantageously be used in an aircraft turbojet engine, for example in order to form the internal wall of the air intake and of the thrust reverser.
In aircraft turbojet engines developed during the last couple of years, noise reduction is one of the priority goals. Indeed, present regulations relative to noise level around airports impose a threshold not to be exceeded. Beyond this threshold, airlines are under obligation to pay financial penalties to the airport authorities.
For this purpose and as FIG. 1 of the appended drawings illustrates this in a sectional view and very schematically, it is common practice to produce the major portion of the inner wall 1 of the air intake of a turbojet engine, as well as the walls 2 of the thrust reverser as noise reduction sandwich panels.
These noise reduction sandwich panels comprise, from the surface of the panel turned towards the outside, a more or less air permeable resistive layer, a waffle core most frequently having a structure of the honeycomb type, and a rear total reflector.
In this conventional layout of noise reduction sandwich panels, the resistive layer has a dissipate role. When a sound wave crosses it, viscous effects are produced which transform acoustical energy into heat.
The height of the waffle core enables the panel to be tuned to the characteristic frequency of the noise to be damped. Dissipation of noise in the resistive layer is maximum when the height of the cells of the waffle core is equal to a quarter of the wavelength of the noise frequency to be damped. The cells of the waffle structure then behave as waveguides perpendicular to the surface of the panel, which give them a response of the xe2x80x9clocalized reactionxe2x80x9d type. The cells form an assembly of quarter wave resonators in parallel.
The rear reflector produces total reflection conditions absolutely required for obtaining the behavior described above of the waffle core.
A noise reduction sandwich panel implanted in an aircraft turbojet engine should meet various requirements of an acoustical, mechanical, structural and aerodynamically nature. For fulfilling the noise reduction function, there are presently different types of sandwich panels.
In the so-called xe2x80x9cnon-linear one-degree-of-freedomxe2x80x9d panels, the resistive layer consists of a metal or composite perforated layer. This structure has the advantage of providing good control of the percentage of open surface, of exhibiting good structural strength and of being easy to produce. However, it exhibits a strong acoustical non-linearity as well as a strong dependence of the strength on the surface tangential flow velocity. Further, as the frequency damped by each cell depends on its depth and as all the cells of the panel have the same depth, the range of frequencies damped by such a structure is limited. In addition, as the resistive layer is in a composite material, the structure has low erosion resistance.
So-called xe2x80x9clinear one-degree-of-freedomxe2x80x9d noise reduction sandwich panels are also known. In this case, the resistive layer is a microporous layer for example consisting of metal fabric, of perforated sheet metal associated with an acoustical fabric or of a metal fabric associated with an acoustical fabric. Such a structure enables the acoustical resistance to be adjusted by changing the components of the microporous layer. Its effective frequency range is reasonable. It has a low to moderate non-linearity as well as a low dependency of the acoustical resistance on the surface tangential flow velocity.
However, the production of a linear one-degree-of-freedom sandwich panel is more complicated than that of non-linear one-degree-of-freedom panel, because the resistive layer comprises two constituents. If the components or the assembly methods are not under control, the structure may have areas of acoustical non-homogeneity and there are also risks of delamination of the resistive layer. Further, risks of corrosion of the resistive layer impose an additional constraint as to the choice of the materials used. Finally, the assembly method of such a panel is lengthy and costly.
xe2x80x9cTwo-degrees-of-freedomxe2x80x9d noise reduction sandwich panels are also known. Such a panel comprises, in addition to a perforated resistive layer and a rear reflector, two superimposed waffle cores, separated by an intermediate resistive layer called a xe2x80x9cseptumxe2x80x9d, which is generally microporous.
As compared with other types of sandwich panels, the panels with two degrees of freedom have a larger range of damped frequencies, a possibility of adjusting the acoustical resistance by means of both resistive layers, and a low to moderate acoustical non-linearity. However, areas of acoustical non-homogeneity appear because of misalignment of the cells from both waffle cores which inevitably occurs upon forming the panel. There are also parasitic transverse propagation phenomena in the areas where the cells of both waffle cores are not aligned. Finally, the assembly method for a panel of this type is lengthy and costly, as the different components of the structure must be assembled one by one.
Various solutions have been suggested in order to overcome the drawbacks of the two-degrees-of-freedom panels resulting from misalignments of cells from both waffle cores.
Thus, in document GB-A-2,252,076, a sandwich panel with two degrees of freedom is obtained from a waffle core produced as a single piece. The intermediate resistive layer is obtained by positioning a separation sheet on one face of this core and by pressing down on the sheet in such a way that it is cut out into pieces having the dimensions of the cells, by the edges of the walls of the latter. These pieces are then pushed in and then stuck in the cells in a predetermined position. However, the problem of accurately placing the different pieces, repeatedly and reliably, is not solved in this document.
Document GB-A-2,098,926 describes a method for integrating a separation sheet into a sandwich panel comparable to the one described in document GB-A-2,252,076. More specifically, this document suggests the use of a press for cutting out the separation sheet to the dimension of the cells by means of the waffle core. As soon as this operation is completed, the waffle core incorporating the pieces cut out of the separation sheet is placed in a bath of dense liquid, the depth of which is carefully monitored in order to push these pieces into their predetermined definitive position. With this technique a two-degrees-of-freedom sandwich panel may be obtained, wherein the cells are aligned properly. However, this method is relatively lengthy and delicate to implement and may prove to be dangerous because of the use of a liquid such as mercury. Further, it is completely unsuitable in the case of a non-planar sandwich panel.
In document GB-A-1,463,918, a two-degrees-of-freedom sandwich panel is obtained by using a single waffle core, the cells of which are divided, in the direction of height, into two subcells by separative components mounted in each cell. In all the described solutions, the separative components have the same hexagonal shape and the same dimensions as the cells in which they are received.
Further, among the various solutions proposed in document GB-A-1,463,918, certain propose joining up several separative components received in aligned cells. These solutions seem advantageous from an industrial point of view, as they lead to a reduction of the setup time for the separative components, which is all the more significant as the number of cells of a sandwich panel is generally very large. More specifically, according to document GB-A-1,463,918, the adjacent separative components are joined up either by a ply which overlaps the upper edge of the partition separating the cells, or directly by an edge-to-edge joining of the separative components, crossing a machined notch in the upper half of the partition, so that its upper edge is at the same level as the separative components.
However, the implementation of the various techniques described in document GB-A-1,463,918, practically gives rise to practically insurmountable difficulties.
Indeed, the implementation of the separative components described in this document would be practically impossible because of the variations in the sizes and shapes of the cells which inevitably occur on a sandwich panel of a large dimension and of a non-planar shape. Thus, taking the small size of the cells into account, their partitions are relatively stiff and practically do not allow the differences in shapes and sizes to be compensated upon the placing of the described separative components, the shape and dimensions of which are the same as those of the cells. This phenomenon is more pronounced when several separative components are joined up to each other. Indeed, the proposed solutions in document GB-A-1,463,918, do not allow the distance between the neighboring separative components to be varied, to take into account the manufacturing tolerances of the honeycomb structure.
Moreover, in the embodiment according to which two separative components placed in two neighboring cells are directly joined together through their adjacent edges and are supported on the upper edge of a notch cut out in the partition separating these cells, the repeated cutting out of this partition on the whole of the panel lowers the mechanical strength of the latter in such an unacceptable way.
The object of the invention is specifically a noise reduction sandwich panel having a structure of the two-degree-of-freedom type integrating inserted separative components designed so that they may be easily mounted, on an industrial scale, into the cells of a unique waffle core, however without causing a reduction in the mechanical strength of the panel or a lowering of its acoustical properties.
According to the invention, this result is obtained by means of a noise reduction sandwich panel, comprising:
a resistive layer;
a reflector;
a unique waffle core, in a single piece, placed between the resistive layer and the reflector, and forming a plurality of juxtaposed cells separated by partitions;
inserted separative components in the cells in order to divide the latter, in the direction of height, into at least two subcells;
characterized in that each separative component has a substantially circular external peripheral edge and in that said panel further comprises positioning units for positioning the separative components within the cells, said positioning units being secured to the separative components and being supported on the edges of the partitions adjacent to the reflector or to the resistive layer.
In a thereby formed sandwich panel, the separative components provide the structure with a layout of the two-degrees-of-freedom type, by the circular shape of the contour of the separative components, the mounting of these components into the cells, generally with a hexagonal section, may be achieved without any particular difficulty, in spite of the differences in shapes and sizes of the cells resulting from the honeycomb structure manufacturing techniques and from the optional non-planarity of the panel.
According to a preferred embodiment of the invention, the external peripheral edge of each separative component has a diameter substantially equal to the distance separating two opposite partitions of a cell. The expression xe2x80x9csubstantially equalxe2x80x9d notably takes into account the inaccurateness of the distance between the partitions, resulting from the manufacturing tolerances.
Advantageously, the separative components have the shape of substantially planar disks. The external peripheral of each of the separative components may then be raised towards the reflectors or towards the resistive layer, to further facilitate the mounting.
According to a first embodiment of the invention, each positioning unit then supports only one separative component.
Each positioning unit may then comprise a small plate, placed between the separative component which it supports and the reflector or the resistive layer, substantially perpendicularly to the latter, said small plate comprising two protruding portions, adjacent to the reflector or to the resistive layer, which are supported in the notches formed in the edges adjacent to the reflector and to the resistive layer of both opposite partitions of the corresponding cells.
Alternatively, each positioning unit may also comprise at least two tabs which extend between the separative component which they support and the reflector or the resistive layer, each tab comprising an end folded between the waffle core and the reflector or the resistive layer.
In this case, advantageously, at least one of the tabs completely crosses a cell adjacent to the one which contains the separative component supported by this tab.
According to a second embodiment of the invention, each positioning unit supports several separative components, inserted in aligned cells along a given direction, and integrates, between each pair of consecutive separative components which it supports, at least one tab able to deform along said direction. With this layout, the separative components may be placed easily and quickly, in spite of the possible variations in the pitch of the aligned cells resulting from manufacturing tolerances and from the optional non-planarity of the panel.
In this embodiment, the separative components supported by a same position unit, are advantageously placed in every other cell, in the direction of the alignment of the cells.
In this case, each positioning unit may comprise small plates, each of which is placed between the reflector and the resistive layer and one of the separative components supported by this positioning unit, perpendicularly to the reflector or to the resistive layer. A tab then joins up each pair of consecutive small plates, overhanging a cell which does not contain any separative component supported by said positioning unit. Further, the tab is bent with respect to the direction of alignment of the cells, in order to be able to deform along this direction.
Each positioning unit may also comprise tabs which join up the consecutive separative components and a straight section of which overhangs the cell which does not contain any separative component supported by this positioning unit. The straight section then has a length greater than the distance separating two opposite partitions of the cell, in order to allow for deformation of both bent sections of the tab, joining up the straight section to the separative components contained in the neighboring cells.
According to an alternative of the second embodiment of the invention, the separative components supported by a same positioning unit are placed in adjacent cells, in the direction of alignment of the cells. Each positioning unit then comprises tabs, a straight section of which overlaps the partition separating the adjacent cells, on the reflector or resistive layer side, in order to extend on both sides of said partitions. In this case, two bent sections of each tab join up the straight section to the separative components contained in the adjacent cells.